Various techniques are used to modulate light output from incandescent, gas discharge, and other types of lamps by attempting to analogously match light output to the voltage or energy level of a modulating signal. Much success has been attained with incandescent lamps in particular, due to the ease of powering a positive-resistance device. However, due to the filament heat persistence of the incandescent lamp, it is difficult to track light output quickly and directly with modulation signal input. Plasma type lamps are more suited for this task, as there is no filament heat persistence in the plasma. The difficulties to be overcome in using hot cathode gas discharge lamps are numerous when compared to their incandescent counterparts. In the prior art, several methods of modulating light output from various types of light emitting elements have been proposed, all of which suffer from a number of limitations and disadvantages.
Methods wherein gas discharge lamps are modulated have usually included the use of inductive or inductive/electronic/dimming type ballasts as a part of their modulation output circuitry. This significantly increases the cost, weight, and size of such devices. Such devices with ballasts also rely on the classic continuous grounded fixtures to initially ionize the lamp gasses, restricting the total viewing angle to 180 degrees or less. In other prior art wherein ballasts have been eliminated, there is no provision for heating lamp cathode filament(s) by a method other than lamp ionization current. For systems that heat lamp cathode filament(s) solely by lamp ionization current, the power necessary for heating lamp cathode filaments and causing electron emission from the lamp cathode filaments must be provided by a minimum value of lamp ionization current multiplied by the "cathode fall" voltage. That minimum ionization current value, as specified by -as discharge lamp manufacturers, is near 1/2 of the maximum rated lamp current.
This leaves little range for continuous, reliable modulation of the light output below that minimum level. When the ionization current decreases to below the minimum necessary to keep the electrodes hot enough to continue to emit electrons, lamp ionization current is extinguished. Consequently, systems which provide heating of lamp cathode filaments solely by lamp ionization current have a modulation range that is limited to the upper half of the lamps light output range. Due to the logarithmic response of the eye to light, the changes in light output that could be made in such a system would be only moderately perceptible.
Many Prior Art apparatus include the use of a diode in the final filter stage or modulation stage. Not only does this arrangement restrict the modulation circuit's use to just one excursion of the modulation signal, it also limits the dynamic range of any modulation signal processed. Thus, if the voltage output of the last filter stage is 6.0 VAC, the dynamic modulation range is restricted to 6.0 VAC divided by 0.6 VAC (the forward diode drop), or 10 times, or 20 dB (voltage). This is due to the fact that the output of the final filter stage or output buffer must first exceed the diode drop before modulation begins to occur.
Many such apparatus require a high input modulation signal voltage for proper light modulation, and some provide no automatic level control. Also in the prior art, no provision is made to utilize the modulation signal for anything other than automatic gain control, and input to the modulation circuitry. No provision for a power-save feature is present in prior art references using ballasts.
Some apparatus provide no means to increase the ionization potential beyond line voltage, restricting the length of lamp that can be ionized. Many do not provide a means for minimally sustaining lamp ionization current, or provide no internal operating current source for the lamp's main ionization current requirements.
Much prior art provides means for modulation wherein one leg of the AC mains is connected to low voltage ground. This creates a "hot ground" and a potential for problems should breakdown occur.
In the Prior Art which lacks the use of a ballast of any kind, and in which infinite analog modulation occurs, such apparatus suffer from the disadvantage of undesirable power dissipation in the power switching component which sources ionization current to the gas discharge lamp. This is due to the lack of the apparatus' ability to decrease ionization voltage to the lamp as the ionization current through the lamp increases. This would be the normal function of the ballast.